demo.cs

public void ConfigureServices(IServiceCollection services) 
{
  services.AddDbContext<AppDbContext>(options => 
options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")
);

services
  .AddMvc()
  .AddJsonOptions(
     options => options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
  )
 SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
 services.AddScoped<Repository>();
 services.AddScoped<Seeder>();
}